Make level configurable for NewStdLog. (#439)

[delicb]

With this change new function is added called NewStdLogAt. It has same
behaviour as NewStdLog, but it allows providing log level to use by
returned *log.Logger.

Couple of things to note:

• it would be much more straight forward to implement this if there is function *zap.Logger.Log(Level, string, …zapcore.Fields) in addition to Debug, Info, etc. I understand if you do not want to introduce it, but if you are opened to it, I can add it and adapt implementation.
• loggerWriter is changed to accept function to use, so that decision which function should be used is elsewhere. Existing code is compatible, since default was Info and that is passed. This can be other way around - loggerWriter can have both logger and level and decide which function to call during Write execution, but I do not really like idea of executing switch case every time Write is called.
• Tests are refactored a bit, so that common parts among new tests and existing one TestNewStdLog can be reused. It might be possible to generalize this to table-driven test, but DPanic, Panic and Fatal levels have special handling, so it might be a bit long. I prefer it the way I wrote it, but I am opened to changing it to table driven test if that is preferred.

[CLAassistant]

All committers have signed the CLA.

[codecov]

Codecov Report

Merging #487 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #487      +/-   ##
==========================================
+ Coverage   97.16%   97.19%   +0.02%     
==========================================
  Files          37       37              
  Lines        2221     2243      +22     
==========================================
+ Hits         2158     2180      +22     
  Misses         56       56              
  Partials        7        7

Impacted Files	Coverage Δ
global.go	100% <100%> (ø)	⬆️

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[akshayjshah]

@delicb Sorry for the long delay reviewing this! Changes and test coverage look great. I'm going to push a commit with a few small nits (e.g., assert.Nil(t, err) is better as assert.NoError(t, err)) and then merge this.

Thank you for the contribution!

[delicb]

@akshayjshah Ah, sure, NoError is better. I actually found out that it exists few days ago (after I created this pull request). I can fix that, if you would like.

Thank you for accepting PR.

[akshayjshah]

🤦‍♂️ I completely forgot about this - I'm so sorry, @delicb! I'll merge this today.

[djui]

@akshayjshah Was there a specific reason to not additionally make this change (log level configurable) for redirectStdLog, or just to stay focus on the PRs description? Any plans on adding it?

[akshayjshah]

Nope, you're welcome to open a PR with RedirectStdLogAt. The existing function can't start taking extra parameters without breaking backward compatibiliy.

[djui]

@akshayjshah Opened #508.